A structure of providing an electron block layer in the interface between a p-type GaN based guide layer and a p-type cladding layer, and the structure of providing the electron block layer into the p-type GaN based guide layer on the active layer are disclosed (for example, refer to Patent Literature 1 and Patent Literature 2).
As shown in FIG. 2, a conventional nitride based semiconductor device 260 which has the structure of providing the electron block layer 225 in the interface between a p-type GaN based guide layer and a p-type cladding layer includes: a GaN based semiconductor substrate 210; an n-type cladding layer 214 placed on the GaN based semiconductor substrate 210; an n-type GaN based guide layer 216 placed on the n-type cladding layer 214; an active layer 218 which is placed on the n-type GaN based guide layer 216, and has MQW (Multi-Quantum Well) structure; an electron block layer 225 placed on the active layer 218; a p-type GaN based guide layer 222 placed on the electron block layer 225; a p-type cladding layer 226 placed on the p-type GaN based guide layer 222; and a p-type GaN based contact layer 228 placed on the p-type cladding layer 226.
Moreover, as shown in FIG. 1, a conventional nitride based semiconductor device 260 which has a structure of providing the electron block layer 225 into the p-type GaN based guide layer on an active layer includes: a GaN based semiconductor substrate 210; an n-type cladding layer 214 placed on the GaN based semiconductor substrate 210; an n-type GaN based guide layer 216 placed on the n-type cladding layer 214; an active layer 218 which is placed on the n-type GaN based guide layer 216, and has MQW structure; a p-type GaN based guide layer 222 placed on the active layer 218; an electron block layer 225 placed on the p-type GaN based guide layer 222; a p-type cladding layer 226 placed on the electron block layer 225; and a p-type GaN based contact layer 228 placed on the p-type cladding layer 226.
In the case of the conventional nitride based semiconductor device shown in FIG. 1, the pattern in which the light from the active layer 218 spreads becomes a shape remarkably pushed out to the n-type GaN based guide layer 216 and the n-type cladding layer 214 side under the influence of the electron block layer 225, and becomes a tendency which also increases threshold current. On the other hand, in the case of the conventional nitride based semiconductor device shown in FIG. 2, the influence by which the pattern in which light spreads is remarkably pushed out to the n-type GaN based guide layer 216 and the n-type cladding layer 214 side can be reduced, and the threshold current is also reduced. However, under the influence of the electron block layer 225 with high aluminum (Al) composition, great compressive stress is applied to the p-type GaN based guide layer 222, and the reliability degrades. In the conventional nitride based semiconductor device shown in FIG. 1, since the electron block layer 225 becomes appearance inserted into the p-type GaN based guide layer 222, the stress occurred under the influence of the electron block layer 225 is alleviated.
As shown in FIG. 3, another conventional nitride based semiconductor device 260 which has a structure of providing an electron block layer 225 in the interface between a p-type GaN based guide layer 222 and a p-type cladding layer 226 includes: a GaN based semiconductor substrate 210; an n-type GaN based buffer layer 212 placed on the GaN based semiconductor substrate 210; an n-type cladding layer 214 placed on the n-type GaN based buffer layer 212; an n-type GaN based guide layer 216 placed on the n-type cladding layer 214; an active layer 218 which is placed on the n-type GaN based guide layer 216, and has MQW structure; the p-type GaN based guide layer 222 placed on the active layer 218; the electron block layer 225 placed on the p-type GaN based guide layer 222; the p-type cladding layer 226 placed on the electron block layer 225; and the p-type GaN based contact layer 228 placed on the p-type cladding layer 226.
In the conventional nitride based semiconductor device 260 shown in FIG. 3, under the influence of the electron block layer 225 with high aluminum (Al) composition, as shown in FIG. 4, the band gap of the electron block layer 225 which has the width L4 becomes large, great compressive stress is added to the p-type GaN based guide layer 222 which has the width L1, and the reliability degrades.
The stress by lattice mismatching is occurred in the interface T1 between the electron block layer 225 and the p-type GaN based guide layer 222 which are composed of this p-type AlGaN layer, and the interface T2 between the electron block layer 225 and the p-type cladding layer 226. At the time of this stress as a trigger, there was a problem that the defect occurred in the drive of the nitride based semiconductor device 260, and the reliability of the nitride based semiconductor device 260 is degraded.
On the other hand, in the nitride based semiconductor device, when the laser oscillation state is continued, oxidation of the emitting end surface of a laser beam advances, and the emitting end surface of the laser beam deteriorates. When the oxidation of the emitting end surface of the laser beam progresses, an element characteristic deteriorates like the rise of laser driving current, or the decline in slope efficiency.
An example of the optical film applicable to the nitride based semiconductor device etc. which does not deteriorate easily and also does not exfoliate easily is already disclosed (for example, refer to Patent Literature 3). In a GaN based semiconductor element of Patent Literature 3, it has an optical film formed in a resonator surface by sputtering of the Al target on the sputtering conditions of a quick deposition rate in atmosphere including Ar gas and oxygen gas using an ECR plasma method.
About a technology of preventing exfoliation of an ohmic electrode formed in a ridge structure of a compound semiconductor laser structure of having the ridge structure, it is already disclosed (for example, refer to Patent Literatures 4 to 6). In Patent Literatures 1 to 3, in the structure composed of an insulating layer formed in the sidewall part of the ridge structure, and an ohmic electrode formed on the insulating layer, in order to prevent exfoliation of the ohmic electrode, the exfoliation prevention layer composed of metal layers is provided between the insulating layer and the ohmic electrode, respectively.
In the compound semiconductor laser structure, a optical confinement effect is obtained by forming the insulating film on the compound semiconductor layer of ridge shape, and providing refractive index difference for a compound semiconductor layer.
However, when forming the insulating film on the ridge shape and the film deposition apparatus having film formation characteristics with directivity is used, there was a fault that covering to the ridge sidewall is insufficient and then the insulating film exfoliates. As an example of a film deposition apparatus having the film formation characteristics with directivity, an ECR (Electron Cyclotron Resonance) sputter device is mentioned.
Conventionally, since the reactive plasma film deposition apparatus having the directivity is used and therefore the reactivity is scarce in the sidewall part of the ridge, the membranous quality of the insulating film formed is wrong (for example, in ZrO2, the quantity of Zr and O shifts from stoichiometric composition), and the exfoliation of the insulating film of the sidewall part occurs because of this.
If the insulating film exfoliates, it will be linked to the defect of the laser initial characteristic that the balance which optical confinement collapses and the shape of beam collapses.
For example, as shown in FIG. 5, a schematic section structure of the ridge part of the nitride based semiconductor device according to a conventional example includes: a p-type GaN guide layer 222; a p-type superlattice cladding layer 226 placed on the p-type GaN guide layer 222; a p-type GaN contact layer 228 placed on the p-type superlattice cladding layer 226; an insulating film 224 formed on the p-type GaN guide layer 222 and on the sidewall of the p-type superlattice cladding layer 226 and the p-type GaN contact layer 228; and a p-side ohmic electrode 230 formed on the insulating film 224 and the p-type GaN contact layer 228. As the SEM photograph of the ridge part of the nitride based semiconductor device according to the conventional example is shown in FIG. 6, for example, covering to the ridge sidewall is insufficient, and the thing in which the insulating film 224 is exfoliated is observed. The relation between the beam intensity of the nitride based semiconductor device according to the conventional example and the horizontal and vertical beam spreading angle is expressed, for example, as shown in FIG. 7. In particular, the symmetry of a horizontal FFP (Far Field Pattern) is wrong.
Furthermore, a group III nitride semiconductor is a semiconductor using nitrogen as a group V element in a group III-V semiconductor, and aluminum nitride (AlN), gallium nitride (GaN), and indium nitride (InN) are representative cases. Generally, it can express AlxInyGa1-x-yN (where 0<=x<=1, 0<=y<=1, 0<=x+y<=1), and is called an indium nitride based semiconductor or a GaN based semiconductor.
A fabrication method of the nitride based semiconductor for growing up the group III nitride semiconductor on the GaN based semiconductor substrate which applies c plane the principal surface by MOCVD (Metal-Organic Chemical Vapor Deposition) is known. By applying this method, the GaN based semiconductor laminated structure, which has an n-type layer and a p-type layer can be formed, and the light-emitting device using this layered structure can be fabricated.
The laser light source of a purple-blue wavelength of being blue and green are increasingly utilized in the field, such the high density recording to the optical disc represented by DVD, image processing, a medical device, measurement hardware, etc. Such a short wavelength laser light source is composed of a laser diode, which used the GaN based semiconductor, for example.
The GaN based semiconductor element grows up the group III nitride semiconductor on the GaN based semiconductor substrate, which applies c plane the principal surface by the MOCVD method, and is fabricated. More specifically, as for the conventional nitride based semiconductor device, for example, an n-type cladding layer composed of an AlGaN single film or an AlGaN/GaN superlattice structure, an n-type guide layer composed of InGaN (or GaN), an active layer composed of InGaN (luminous layer), a p type guide layer composed of InGaN (or GaN), an electron block layer composed of AlGaN, an AlGaN single film or a p-type cladding layer composed of superlattice structure of AlGaN/GaN, a p-type contact layer composed of an AlInGaN layer etc. grow sequentially by the MOCVD method on the nitride semiconductor substrate, and the semiconductor laminated structure composed of these semiconductor layers is formed. In the active layer, light-emitting occurs by the recombination of the electron injected from the n-type layer and the hole injected from the p-type layer. The light is confined between the n-type AlGaN cladding layer and the p-type AlGaN cladding layer, and is spread in the direction vertical to the laminating direction of laminated semiconductor structure. A resonator edge face is formed in the both terminals of the propagating direction, resonance amplification of the light is performed repeating stimulated emission between the resonator edge faces of this pair, and a part of this is emitted from the resonator edge face as a laser beam (for example, refer to Patent Literature 7, Non Patent Literature 1, and Non Patent Literature 2).
As an active layer, the MQW structure which inserted a plurality of layers into the shape of sandwiches for the well layer by the barrier layer with a greater band gap than the well layer is adoptable (for example, refer to Patent Literature 8).
Generally, since InGaN, GaN, and AlGaN differ in a lattice constant, stress occurs by laminating each layer.
In the nitride based semiconductor device, AlGaN is used as a cladding layer, and also the p-type AlGaN layer of high Al composition is used in order to block the electron from the n-layer. The stress by lattice mismatching occurs on the interface between this AlGaN layer and the GaN layer. As a result, the stress concentrated on the step corner of the stripe for current concentration and optical confinement of the nitride based semiconductor device occurs.
In particular, it is considered that great stress occurs in the region to which the stripe side and the etched bottom crosses (step corner of the stripe) since the stripe region has convex structure for the other region. At the time of this stress as a trigger, a defect occurs in an element drive and the reliability of the element is degraded.
Generally, since InGaN, GaN, and AlGaN differ in a lattice constant, stress occurs by laminating each layer. As a result, curvature occurs in the nitride based semiconductor device. The more the thickness of the nitride semiconductor substrate becomes thin, the more this curvature appears remarkably. If the curvature is large, a crack becomes easy to occur and becomes a cause of yield rate reduction.
When fabricating the nitride based semiconductor device, by applying element size small, a number of getting per wafer can be increased and the cost can be reduced. Since a cleaved surface is used as a resonator mirror when fabricating the nitride based semiconductor device, it is necessary to form a clear cleaved surface. When fabricating the nitride based semiconductor device with a small chip size, in order to obtain a clear cleaved surface, it is necessary to apply substrate thickness thin. However, if substrate thickness is applied thin, the curvature of the substrate appears remarkably as mentioned above.
Citation List
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